The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the adjustment of receiver gain to obtain low-noise images under varying signal conditions.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.2, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received, digitized and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.x and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The amplitude of the received NMR signal used to reconstruct an image will vary greatly depending on a number of factors. For example, the NMR signal amplitude will increase with increased slice thickness, increased pulse repetition time (TR), decreased echo time (TE) increased patient size, increased fat content in patient and choice of receiver coil (i.e. whole body coil, surface coil, head coil, etc.). These factors remain relatively constant during the scan and the receiver gain is typically set to a single value during a prescan process, which insures that the peak NMR signal amplitude will not over-range the analog-to-digital converter. Such a prescan is disclosed, for example, in U.S. Pat. No. 4,806,866 entitled "Automatic RF Frequency Adjustment For Magnetic Resonance Scanner".
The quality of the reconstructed image is related to its signal-to-noise ratio (SNR), and this SNR may be degraded when the receiver gain is lowered to handle the largest signals. This occurs because the noise figure within the MRI system receiver increases as the receiver gain decreases. During a typical scan the NMR signals from different views will have a range of amplitudes and the receiver gain is fixed at a value which utilizes the full range of the analog-to-digital converter when the maximum expected signal amplitude is received. This means that many low level NMR signals are acquired during the scan without utilizing the full range of the analog-to-digital converter, yet have less than the optimal SNR figure. For example, views with minimal phase encoding gradients applied have high amplitudes and must be acquired with low system gain yielding a less than optimal noise figure. The views acquired with high phase encoding gradients, on the other hand, have small amplitudes, but are acquired with a system noise figure set for the largest signal conditions.